This disclosure relates to improvements in hearing assistance devices through the use of active noise reduction.
Hearing assistance devices, such as hearing aids and personal sound amplification products (PSAPs), as well as some conventional or specialized headphones, detect sound in the environment of a user and amplify it to improve the ability of the user to hear it. Hearing aids, in particular, may adjust the character of the amplified sound based on the unique hearing loss profile of the user. PSAPs and headphones may also be personalized. To a large extent, the distinction between a hearing aid and a PSAP is one of intended use determined in part by marketing—PSAP and hearing aid features may be added to conventional headphones or specialized headphones, such as tactical headphones, through internal or external software, or through hardware. The terms “hearing assistance device,” “headset,” “earphone,” and “headphone” in this disclosure refer to any such product, without regard to the regulatory status or marketing position of the product. The terms are also not meant to limit the physical form-factor of the product, though certain examples may only apply to some form-factors.
Active noise reduction (ANR) headsets typically employ either feedback or feed-forward ANR, or both. Feedback ANR is accomplished by filtering a signal from a microphone coupled to the ear canal through a control loop, then outputting that signal through a loudspeaker (typically referred to in the context of hearing aids as a “receiver”). Feedforward ANR is accomplished by filtering a signal from a microphone on the outside of the earphone through a filter, then outputting that signal through a loudspeaker. The signals output by the loudspeaker in either arrangement destructively interfere with acoustic signals reaching the ear canal through passive paths, i.e., through the head, through the headphones, or around the headphones, and reduce the total acoustic energy reaching the ear drum.